allowed memory size

Why does this happen and how can i fix this?

Fatal error: Allowed memory size of 67108864 bytes exhausted (tried to allocate 30720 bytes) in /mysite.com/mm/wp-content/plugins/buddypress/bp-forums/bbpress/bb-includes/functions.bb-template.php on line 233

I tried to fix it 2 weeks ago and ended up reinstalling everything. i put too much work into now and just can't do that again...

I have read other posts about changing settings in php.ini file and wp-settings.php file but I don't have a php.in file and my hosting company says i should just create one... but I am not sure what to put in it and I don't know what or where to add the memory settings in wp-settings.php...